The invention relates to a process for cleaning a stream of crude gas or waste gas charged with hydrocarbon vapors, gasoline vapors in particular, resulting in recovery of the hydrocarbons or gasoline.
During the refueling of tanker trucks, oil tankers, or storage tanks and the like, with inflammable fuels for example, a hydrocarbon-air mixture is formed above the liquid involved and is displaced in the tank by the liquid during the filling process. Unless special precautions are taken, this mixture escapes into the atmosphere. Processes for recovering hydrocarbons from a hydrocarbon-air mixture have been proposed for the purpose of reducing the resulting economically significant losses of hydrocarbons and of abating environmental pollution.
As examples, DE-PS (German Patent) 27 43 188 and DE-PS 27 60 187 describe processes for recovery of light hydrocarbons from a mixture of air and hydrocarbons. In these processes, the hydrocarbon-air mixture is forced through a single-stage adsorber system, the hydrocarbon components being largely adsorbed from the hydrocarbon-air mixture. Two single-stage adsorber systems are connected in parallel, so that, over the period during which the hydrocarbon-air mixture is forced through one adsorber by slight excess pressure, the second adsorber, which already contains adsorbed hydrocarbons, is regenerated under negative pressure. The adsorbed hydrocarbons are desorbed as a hydrocarbon-air mixture, and exhausted by means of a vacuum pump. The mixture is then separated into a liquid and a vapor phase in a separator stage, and the vapor phase is forced through an absorber in which liquid hydrocarbons as absorbent flow in the direction opposite that of the vapor phase containing hydrocarbons, the gaseous hydrocarbons being mostly absorbed or liquefied. The inert gases (air) that cannot be condensed emerge from the absorber at the head under a hydrocarbon charge of approximately 2 kilograms per kilogram of air, and are returned to the flow of crude gas, where they are subjected to treatment again together with the crude gas in a single-stage adsorber.
In these prior art processes, considerable additional adsorber capacity must be kept ready in order to recover the amounts of hydrocarbons returned from the absorber head. Another disadvantage of these prior art processes is that the desorption must be very carefully executed so that the single-stage adsorber downstream will be regenerated as completely as possible so that the adsorber will be able to adsorb sufficiently even difficultly adsorbable components such as propane during the next adsorption phase. Only in this way, is it possible to bring about the required reduction of the hydrocarbon-air mixture at the adsorber head. This can, of course, be accomplished only if the suitable operating conditions are carefully adhered to. First and foremost, a very good vacuum must be created in desorption, something that requires very high vacuum pump performance. In addition, stripping gas must be introduced, this resulting in return of a greater amount of hydrocarbons to the stream of crude gas.
A similar process is disclosed in U.S. Pat. No. 4,261,716, in which two single-stage adsorbers are connected in parallel, so that the hydrocarbon-air mixture can be forced through one of the adsorbers while the other is being regenerated. Inasmuch as the conditions under which the process is executed are very similar to those of the processes referred to in the foregoing German patents, the same disadvantages as described previously result.
In addition, DE-PS 32 10 236 describes a process for cleaning a waste gas stream charged with vapor and/or gaseous pollutants. The initial hydrocarbon charge of the waste gas stream to be treated is first reduced by means of an upstream scrubbing stage, in which high-boiling or difficultly desorbable components are precipitated before they enter the adsorbers. This avoids blockage of the adsorbers by the high-boiling components, so that the operating capacity of the adsorbers is fully available for adsorption of the remaining pollutants. A disadvantage of
energy-intensive and cost-intensive, and that under unfavorable conditions can cause icing problems.